Page 1 of 3

Christmas Comes Early!

Posted: Thu Jun 08, 2017 2:40 pm
by Garrett Young
(Note: I have no affiliation with the seller and can't guarantee the functionality of the listed item but I purchased one and will update the thread with my findings)

Spellman PTV50N200 (related thread: viewtopic.php?f=11&t=4852)

-50kV 4mA 200W (can be boosted to 7 mA/350W using the schematics in the linked thread)

http://www.ebay.com/itm/Spellman-50kv-H ... 2706614602

$125! (now $150)

Re: Christmas Comes Early!

Posted: Thu Jun 08, 2017 3:59 pm
by ian_krase
Oh wow.

Is that enough current though?

(Who cares, I'm not doing fusion for the *fusion*)

Re: Christmas Comes Early!

Posted: Thu Jun 08, 2017 4:06 pm
by Garrett Young
In my device, 50kV and 7mA would produce very easily detectable fusion.

Re: Christmas Comes Early!

Posted: Thu Jun 08, 2017 6:13 pm
by Garrett Young
Here are the changes that I identified to convert to the 350W version:

C15 0.047/50V

C39 0.022/50V
R50 4.7k 5%

JB5 IN (Used for Fan 24VDC)

R26 20k, 22T (?, table slightly unclear)

Re: Christmas Comes Early!

Posted: Thu Jun 08, 2017 6:24 pm
by Dennis P Brown
But your chamber operates at 33 milli-torr which is rather high compared to standard fusors (5 milli-torr); having a small 4-way "T" as a chamber is an apparent advantage and might be the best approach. So using 50 kV and 7 ma on a normal sized chamber might not work. I like your approach better but if someone wants to use a 350 watt PS might it might be best for them to follow your complete design.

Re: Christmas Comes Early!

Posted: Thu Jun 08, 2017 11:21 pm
by ian_krase
Interesting.

I wonder if I can make an accelerator.

Re: Christmas Comes Early!

Posted: Fri Jun 09, 2017 2:35 am
by Sarvesh Sadana
Would it not be possible to purchase two power supplies and run them in parallel, giving you 50 kv at 14 ma?

Re: Christmas Comes Early!

Posted: Fri Jun 09, 2017 5:46 am
by John Futter
Sarvesh
This usually ends up with two lots of smoking ppwer supply remains

Re: Christmas Comes Early!

Posted: Fri Jun 09, 2017 12:40 pm
by Richard Hull
John is right, paralleling power sources to get more current tends to work better with batteries and even they need load leveling sharing resistors that waste power.

viewtopic.php?f=11&t=4852

Richard Hull

Re: Christmas Comes Early!

Posted: Fri Jun 09, 2017 3:13 pm
by John Futter
Actually i'm not quite right with the paralleling of supplies
some supplies have the capability to slave to a master supply so that there is only one control loop operating.
If you have access to the internal control loops and know what you are doing it is possible but I would say this is out of reach to most people here

Re: Christmas Comes Early!

Posted: Fri Jun 09, 2017 5:38 pm
by Andrew Seltzman
The spellman power supplies do parallel nicely and will not sustain damage. I have paralleled 2 of the 40kV version on my fusor, see:
viewtopic.php?f=6&t=10294&start=100

One thing to remember is that these supplies are not stiff voltage sources, they are cockroft-walton multipliers; current only flows in one direction. If you parallel two of them and set them to different voltages, the output voltage will be the one of the supply set to greater voltage with that one supplying all the current. In practice, running them in constant current mode will definitely work in all conditions, but I have also found that by connecting the current program line of the "slave" supply to the current monitor line of the "master" supply and running both voltage program lines at the same voltage set point will cause both supplies to current share equally. You may need to adjust the slave supply to a slightly higher voltage to make this work, but in my case I did not need to do so.

The control loops of these supplies could be modified easily, but there is no need to do so.

Re: Christmas Comes Early!

Posted: Sat Jun 10, 2017 1:15 am
by ian_krase
Interesting. Though still not something I would try with my own supplies.

There's another result of them not being "stiff": they take time to change between voltage and current mode, and can output a much higher pulse current then the control is set to. This means they can electrocute you even if the current is set to a "safe" level, and it also means that, unlike, say, an NST, they aren't good for arcing because the pulses of very high current heat stuff up.

Re: Christmas Comes Early!

Posted: Sat Jun 10, 2017 4:21 am
by Andrew Seltzman
They do have filter capacitors on the output and thus can output a higher current pulse during an arc, but the energy storage is small. That being said since they can output 5mA or more continuously they can electrocute you, pulse or not. It's never good to intentionally arc switching power supplies continuously at a high rate, but this is not the normal operation in a fusor unless you are doing it intentionally. The PTV series of supply are an ideal candidate for a fusor, and I would recommend this model, you could probably get near 1e6n/s once it is modified for 350W.

Re: Christmas Comes Early!

Posted: Sat Jun 10, 2017 2:38 pm
by ian_krase
Do you still need a ballast resistance?

Re: Christmas Comes Early!

Posted: Sat Jun 10, 2017 3:20 pm
by Andrew Seltzman
I've never used one with my switching power supplies.

Re: Christmas Comes Early!

Posted: Sat Jun 10, 2017 3:30 pm
by John Futter
Ian
Unless you can guarentee that you will not have plasma breakdowns then I would use a ballast to limit dI/dt excursions in the psu
I will put up a photo of what happens to a multiplier stack if you let the supply continuously collapse the output voltage.
the first components to complain are the multiplier capacitors usually somewhere near the centre of the multiplier

Re: Christmas Comes Early!

Posted: Sat Jun 10, 2017 4:03 pm
by Andrew Seltzman
Ian,

As John said, the word is "continuous"; an occasional spark won't harm the supply, but if you are continuously arcing(like surface tracking on your ceramic grid insulator) the supply may be damaged. More advanced supplies designed for x-ray tubes, like the DXM series, have an arc fault counter that will shut down the supply after a given number of arcs in a time window. The PTV does not have this feature (it does have current limiting), so don't leave it arcing.

I tend to favor not using a ballast resistor since it complicates the HV component train by adding another series component between the supply and grid which then needs to be insulated as well resulting in a larger more complex system. Also consider cooling issues, lets say you want a 100k ballast resistor and have a 10mA system at 40-50kV. At 50kv, an arc/short will draw a peak current of 500mA from the capacitor. During normal operation, the resistor will drop 1kV at 10mA dissipating 10W. It's not a lot of power, but if in an enclosed/insulated system it may overheat. Also consider an RL snubber (parallel resistor and inductor), that way the resistor will snub the di/dt spike, but the inductor will provide very low DC resistance for normal operation.

It's not that hard to include either one, but then again my fusor runs very stably, arcing isn't a major concern for me, and I haven't had any problems.

Re: Christmas Comes Early!

Posted: Sat Jun 10, 2017 4:15 pm
by ian_krase
How would you make the inductor? Coil of HV wire?

Re: Christmas Comes Early!

Posted: Sat Jun 10, 2017 4:45 pm
by Andrew Seltzman
I've had good success wrapping copper magnet wire directly around the resistor from terminal to terminal. This was a snubber for a 100kV, 50A system though and the resistor was about 2ft long 1.5" dia, for a smaller resistor for a fusor you might need better insulation to prevent flashover between the windings.
IMG_20150901_134527848.jpg
IMG_20150901_134536374.jpg

Re: Christmas Comes Early!

Posted: Sat Jun 10, 2017 5:12 pm
by ian_krase
Cool. Makes sense, I guess. Does the assembly get potted or oil-onsulated, or just stay in air?


(And also what values would you use for a current controlled switched supply, contrasting with the faq which is for transformers​ mostly)?

Re: Christmas Comes Early!

Posted: Sat Jun 10, 2017 5:14 pm
by Andrew Seltzman
In that design, it was in an insulating tube in air. For your fusor you could go either way

Re: Christmas Comes Early!

Posted: Sat Jun 10, 2017 7:42 pm
by Garrett Young
Andrew,

The RL snubber is a good suggestion. I'm calculating something on the order of 100mH in parallel with a 100kohm (5us short, 50kV, and 10nF output capacitance) to limit the current pulse to 3A. Does this make sense to you?

Re: Christmas Comes Early!

Posted: Sat Jun 10, 2017 8:07 pm
by ian_krase
That's a very large value resistor, for a big ceramic one.

Re: Christmas Comes Early!

Posted: Sun Jun 11, 2017 1:34 am
by ian_krase
And also a pretty big inductor. Seems like this might need to be made with smaller components under oil or paraffin.

Re: Christmas Comes Early!

Posted: Sun Jun 11, 2017 1:09 pm
by Richard Hull
VCU (Virginia Commonwealth University) had a major issue with there grossly under powered, yet very expensive, Spellman supply on their fusor. They had to carefully select a ballast resistor due to the current limit tripping out the system when the plasma struck. Ultimately, they hit the correct value.

They have a 0-30kv 10ma model $$$$. They are limited to running at about 25 kv due to the gobbling up of voltage in the ballast. Thus, their fusion is minmal to marginal. Their good detection system was appropriated for some unknown reason and they were givien a virtually useless battery powered neutron detector of a type useful only in a real neutron field near a reactor. A complex scenario to be sure. They never seem to ask for assistance until they are up against it and it is late in the senior year. The effort is a continuing senior only project.
Thus, each new team must start cold. Not good. U of W has it right....Bring in all classes as pure volunteers letting the freshman be step-and-fetchits, machinists and low level techs. Each ascending class is allowed to become more involved at higher levels of operation and finally research.

Richard Hull